1. Field of the Invention
The present invention relates generally to apparatus and methods for determining the instantaneous xe2x80x9ctruexe2x80x9d depth of a well logging tool suspended from a cable inside a well bore. More particularly, the present invention relates to apparatus and methods for determining the true tool depth corresponding to formation data measured by the tool while it is being raised or lowered inside a well bore. Still more particularly, the present invention relates to apparatus and methods for determining the true tool depth in real time from z-axis accelerometer and cable tension measurements.
2. Description of the Related Art
Well logging is a method of gathering information about subsurface formations by suspending a measurement tool from a cable, such as a wireline, within a well bore and raising or lowering the tool as it makes measurements along the well bore. For purposes of data analysis, it is important to accurately associate the true depth of the tool with the measurement sample obtained at that depth.
In a typical wireline logging system, the depth of the logging tool is taken to be the measured wireline length spooled out at the surface. The wireline length is measured by a sheave wheel, called a depth encoder, that rotates as the wireline is lowered or raised by the winch. A shaft disposed internally of the depth encoder generates pulses with each rotation, and these pulses are counted by the depth system to determine the length of wireline spooled out. The wireline is marked at regular length intervals with magnetic markers that are also counted by the depth system to correct for any accumulated errors. Finally, the depth system corrects for wireline stretch due to gravity.
Typically, logging signals are sampled downhole at fixed time intervals as the tool is being raised by the wireline at a constant speed. As samples are taken subsurface, the data samples are transmitted to the surface through electrical conductors in the wireline. The surface system then xe2x80x9cresamplesxe2x80x9d the data, or in other words, converts the time-based data into depth-based data using fixed increments of depth as measured by the depth system at the surface.
This resampling process is accurate as long as the tool is moving at the same speed as the wireline at the surface. However, due to the combination of cable elasticity and irregular tool motion caused by friction between the tool and the well bore wall, the instantaneous speed of the tool may differ substantially from the speed of the wireline at the surface. For example, when there is a moderate amount of friction between the tool and the bore hole wall, slight slipping may occur, and cable elasticity may lead to tool bouncing, commonly called the xe2x80x9cyoxe2x80x94yoxe2x80x9d effect, or stick and slip movement. This yoxe2x80x94yo effect intensifies when there is sufficient friction to stop the tool until wireline tension overcomes static friction and tool motion resumes. When the speed of the tool differs from the speed of the wireline at the surface, the surface depth measurement will not match the true depth of the tool.
Speed correction is one known process for correcting the wireline depth recorded at the surface for irregular tool motion. The input for traditional speed correction software is data recorded by an accelerometer within the tool that has a measurement axis aligned with the axis of the tool. This is called a z-axis accelerometer. Straightforward integration of the accelerometer data is one speed correction method. Kalman filtering as described in U.S. Pat. No. 4,545,242 to Chan is another speed correction method. Both methods attempt to determine when the tool is stuck and to correct the measured tool depth accordingly. However, conventional speed correction algorithms have some shortcomings, such as difficulty in correctly identifying stuck intervals.
A further shortcoming conventional speed correction algorithms is as follows. Traditional speed correction methods are applied to correct the resampled, depth-based data, thus resampling the data a second time as a function of equal increments in the corrected depth. This poses several problems. First, logging tool strings comprise several different types of sensors having measurement points that are displaced from the bottom of the tool at various distances. The first depth-based resampling process will depth align these measurements before saving them in the database. Also, many measurements are averaged with spatial averaging filters to remove statistics or spatially deconvolve the data. However, traditional speed correction is applied before depth alignment and before any depth-based filtering, which causes duplicative processing.
As an example, speed correction may be used to depth-correct data from imaging tools that record an electrical or acoustic image of the borehole. These devices usually record all data at a single measurement depth, although there are several sensors located at different positions along the length of the tool, thus requiring depth alignment. To properly speed correct the data from these various sensors, the time-based data is first resampled into depth-based data with no depth alignment and no filtering. Then speed correction is applied to resample and depth correct the depth-based data. Finally the depth delays and filtering can be applied, thus requiring all the depth-based processing to be duplicated. To eliminate duplication in data processing, it would be advantageous to have a speed correction method that can be applied to the original, time-based data.
An additional complication is posed when speed correcting the data from non-imaging sensors. When a tool that has been stuck for several tens of seconds breaks free, the speed of the tool downhole can be several times the nominal logging speed. Data sampled during periods of rapid tool motion are sampled at a lower density (in samples per unit distance) than data sampled during periods of steady tool movement. Because speed correction is applied to the resampled, depth-based data, interpolation is required between samples to stretch out the data. However, if speed correction were instead applied to the time-based data before the resampling procedure, it is possible to fill in these low-density sections with actual data samples instead of interpolating. This provides another reason why it would be beneficial to have a speed correction method applicable to the original, time-based data. The improved speed correction system of the present invention overcomes the deficiencies of the prior art.
The present invention features an improved speed correction apparatus and method utilizing measurements from a z-axis accelerometer and measurements of wireline tension at the surface to better identify intervals where the logging tool is stuck against the well bore wall. The wireline tension may be measured by any known means, such as with a strain gauge load cell attached to a pulley at the top of the derrick. The z-axis accelerometer can be located anywhere along the length of the logging tool, as long as the entire tool behaves as a rigid body.
A preferred embodiment of the invention is a well logging system that comprises: a logging sonde, a wireline cable, a distance sensor, a tension sensor, and a surface computer. The logging sonde is configured to move through a wellbore while making logging measurements of the formation around the wellbore. The logging sonde includes an accelerometer configured to measure acceleration of the logging sonde. The wireline cable couples the logging sonde to a reel on the surface. As the reel reels in wireline cable, the distance sensor detects the length of wireline cable that is retrieved. The tension sensor is configured to measure the tension in the wireline cable. The surface computer receives measurements from the accelerometer, the distance sensor, and the tension sensor, and it is configured to determine intervals in which the logging tool is stuck. At least in part, this is done by finding xe2x80x9cslowxe2x80x9d intervals in which the wireline cable is stretching at about the same rate that the wireline cable is being reeled in.
The present invention comprises a combination of features and advantages which enable it to overcome various deficiencies of prior apparatus and methods. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.